Generally, lighting is an activity or a function to brighten a certain place using various kinds of light sources with a particular purpose. Lighting is mostly used to make an environment brighter in the night or in the dark.
FIG. 1 is a cross-sectional view illustrating a flat light device according to an exemplary embodiment of a conventional art. Referring to FIG. 1, the light device according to the exemplary embodiment of the conventional art includes a light source 10 and a louver or a reflecting shade 20. As for the light sources 10, an incandescent light bulb, an LED, a CCFL, or the like may be used Referring to FIG. 1, light at angles denoted with dotted lines causes visually discomfort to a person when it is transferred to the person. Such a lighting device may reduce the UGR mechanically, but cannot be aesthetic or perfect flat lighting.
FIG. 2 is a cross-sectional view illustrating a flat lighting device according to another exemplary embodiment of a conventional art. Referring to FIG. 2, a lighting device 30 includes a light source 10 and a diffusion plate 40 for diffusing light emitted from the light source 10. The light emitted from the light source 10 is discharged to the outside through the diffusion plate 40. The diffusion plate is used for reducing a hot spot of the light source and emitting uniformly light. Even if the diffusion plate 40 is used, as shown in FIG. 2, the light at the angles denoted with the dotted lines still gives discomfort to the eyes of a person. That is, the diffusion plate 40 scatters the light up to a direction in which the UGR is high enough causing fatigue of the eyes due to the glaring. Thus, so such a diffusion plate fails to meet the standard of an indoor flat lighting device.
Accordingly, it is important to reduce the glaring to the eyes in indoor flat lighting. The degree of discomfort due to the glaring to the eyes is represented using a constant called UGR (Unified Glare Rating). That is, the UGR is a value calculated by quantifying the degree of discomfort giving to the user of a lighting device.
The UGR is calculated as the value of a light flux emitted at the angle between 65 deg to 90 deg when a direction facing a bottom surface from a ceiling provided with a lighting device is set to 0 deg and a direction parallel to the ceiling is set to 90 deg. That is, the glaring to the eyes will reduce when the light flux at 65 deg to 90 deg is reduced. In Europe and US, an indoor lighting device has to be less than 19 of UGR.